1. Field of the Invention
The invention relates to an apparatus for converting a sequence of m-bit information words into a modulated digital signal, where m is an integer, the apparatus comprising
input means for receiving the sequence of m-bit information words, PA1 m-to-n bit converter means for converting the sequence of m-bit information words into a sequence of n-bit codewords, where n is an integer larger than m, the sequence of codewords forming the modulated signal, which signal comprises bit cells having a first signal value and bit cells having a second signal value, PA1 determining means for determining a coding state related to a digital sum value at the end of a modulated signal portion, which digital sum value denotes for said modulated signal portion a running value of a difference between the number of bit cells having the first signal value and the number of bit cells having the second signal value, the m-to-n bit converter means comprising selection means for selecting for the conversion a code word from a set of codewords that depends on the coding state, at least one of the digital sum values determining a first and a second coding state, the first and second coding state being determined in response to the information word that corresponds to the previous codeword and to a method for converting a sequence of m-bit information words into a modulated digital signal. PA1 sync word generator means for generating a sync word for a block of p consecutive code words in the modulated signal, the sync word generator means being adapted to generate said sync word from the following set of available sync words: PA1 sync word generator means for generating a sync word for a block of p consecutive code words in the modulated signal, the sync word generator means being adapted to generate said sync word from the following set of available sync words:
The invention further relates to a recording apparatus in which such a conversion apparatus is used.
The invention further relates to the modulated signal obtained and to a record carrier on which the modulated signal is recorded.
The invention furthermore relates to an apparatus for reconverting the modulated signal into a sequence of m-bit information words.
Finally, the invention relates to a reading apparatus in which a record carrier of this type is used.
2. Description of Related Art
A conversion apparatus as defined in the opening paragraph is known from WO 95/27,284, document D1 in the list of related documents that can be found at the end of the description.
Said document describes a modulation system in which a sequence of 8-bit information words is converted to a sequence of 9-bit code words. The sequence of 9-bit code words forms a modulated signal, which signal comprises bit cells having a first or a second signal value, such as a `high` and a `low` signal value representing the logical bit values `1` and `0` respectively in the codewords. Each bit cell represents a bit from the 9-bit codeword sequence, whilst the logical value of the bit is denoted by the signal value of the bit cell. On conversion, each time a 9-bit code word is delivered, together with a coding state, said coding state having a relation with a digital sum value for the delivered part of the modulated signal. This digital sum value denotes the difference between the number of `high` bit cells and the number of `low` bit cells for the delivered part of the modulated signal.
The next code word to be delivered is selected from a set of code words in dependence of the coding state derived with the previous codeword. The code words in the set are selected in such a way that the digital sum value of the modulated signal remains within a small range, which leads to the fact that the frequency spectrum of the signal has reduced frequency components in the low-frequency area. Such a signal is also referenced a DC-free signal or DC-balanced signal. The lack of low-frequency components in the signal generally has great advantages for information transfer via a record carrier or other transmission channels, such as eg. an optical fibre.
The conversion apparatus known from WO 95/27,284 provides for an increased information density on the record carrier.
The known apparatus, however, has the disadvantage that sometimes errors occur upon reconversion, resulting in a distorted reconverted sequence of information words.